Illumination devices for projection applications, which have a phosphor wheel comprising one or more phosphor substances, are known from the prior art, for example from the document US 2010/0245777 A1. These illumination devices comprise a pump light source, which excites the phosphor to emit light with a wavelength different to the pump light wavelength (wavelength conversion of the pump light by means of phosphor). Conventionally, the phosphors are arranged successively in the rotation direction of the phosphor wheel, so that the light (conversion light) emitted by the respective phosphor is generated and delivered to the imaging system in temporal succession.
Lasers, for example laser diodes, are preferably used as the pump light source. In this case, the technology is also known by the term LARP (Laser Activated Remote Phosphor).
The conversion efficiency of a phosphor (=radiation power of the conversion light relative to the radiation power of the pump light) is dependent on the temperature of the phosphor and therefore on the intensity of the pump light on the phosphor. With increasing temperature and intensity, the conversion efficiency of the phosphor decreases. Since the temperature distribution of a phosphor is also dependent on the intensity distribution of the pump light on this phosphor, there is in principle at least an indirect dependency of the conversion efficiency of the phosphor on the intensity distribution of the pump light on the phosphor. The decrease in the conversion efficiency of phosphors with increasing phosphor temperature or pump intensity is referred to as quenching.
The light distribution on the phosphor, for example in a LARP projection application, is generally a compromise between the conversion efficiency of the phosphor and the geometrical efficiency of the optical unit which collects the conversion light from the phosphor and makes it ready for the application, also referred to below as a collecting optical unit. The lower the pump light intensity, the higher the phosphor efficiency. With an increase in the area exposed to the pump light on the phosphor (pump light area), on the other hand the efficiency of the collecting optical unit decreases when the image of the pump light area, generated by the collecting optical unit, is increasingly curtailed by an aperture. For this reason, the size of the pump light area is a compromise in order to maximize the overall efficiency (which is the product of the phosphor conversion efficiency and the efficiency of the collecting optical unit).
Different phosphors have different quenching properties. There are therefore different requirements for the pump light distribution on the phosphors, in order to maximize the overall efficiency. If the phosphors are on a planar phosphor wheel, then with a pump light source which is invariant as a function of time all the phosphors have to date been illuminated with the same pump light distribution. This can entail losses in the overall efficiency of the system, since the product of phosphor conversion efficiency and geometrical efficiency is different for each phosphor on the phosphor wheel.